Since yesterday, January 16th, pacman 4 is in the core repository and upgrading to it breaks the system.

If you can wait with any updates, wait. If you must update some packages, only update those which you need to update right now and which don’t affect the system.

To exclude the packages which surely breaks your system, edit your /etc/pacman.conf and add to IgnorePkg all packages related to any package manager stuff and all packages you know that they break something.

IgnorePkg = pacman package-query yaourt libarchive

If you encounter trouble with other packages, let me know.

If you updated libarchive for whatever reason pacman stops to work while it outputs this error message:

To fix this you have to create a symlink pointing from libarchive.so.2 to libarchive.so.12.0.3. Adapt the version number of according to your installed archive lib. Check this with

ls -l /usr/lib | grep libarchive.so

Now pacman should work again.

Update (Jan 21st):
On my arm-based Archlinux everything went fine when upgrading and generating the keyring. On my desktop I’ve got to fiddle with some configurations but it seems that nothing bigger goes wrong today. Maybe I was completely wrong with my statement “pacman 4 breaks the system” but libarchive really made trouble days ago so it wasn’t a bad choice to wait a little bit.

Luckily pacman upgrade didn’t break my arm. no pun intended. Though it did let worry for a while, since intermediate stage had libarchive already new and pacman still old. During that minute or so pacman was not usable, if I interrupted the upgrade I would have been left with a broken system.
Thanks for the heads up which I read a few keystrokes too late!